This invention applies to paper webs or sheets, and more specifically to tissue or paper tissue webs, that are commonly used in paper towels, napkins, facial and toilet tissues. The important characteristics for such papers (simply referred to as ‘tissue papers’ from this point on) are bulk, softness, absorbency, stretch and strength. There is an ongoing work to improve each of these characteristics without seriously affecting the others. Methods for making conventional wet pressed (CWP) and through-air-dried (TAD) tissue papers are well known in the art. Both types of tissue papers are formed by draining a cellulosic fiber suspension through a forming fabric to create the paper web. The cellulosic fiber suspension is deposited onto the forming fabric by means of a headbox which uniformly deposits the suspension. Depending on machine type, there can be some initial vacuum or centrifugal dewatering of the web. For CWP tissue papers, the web is further dewatered at the pressure roll, where the sheet is pressed between the pressure roll and the Yankee dryer to a typical consistency of 40-45%. Final drying is accomplished by the steam heated Yankee dryer in combination with hot air impingement hoods. For TAD tissue papers the web is further dried by the through-air dryer(s) which force hot air through the web to obtain a typical consistency of 60-85%. Again, final drying is accomplished by the steam heated Yankee dryer in combination with hot air impingement hoods.
Conventional fluff pulp and methods for making such pulp are well known in the art. Important properties include absorbency, burst strength and specific shredding energy. Such pulp is typically made by forming a thick web or sheet on a Fourdrinier wire and subsequently pressing and drying the paper sheet into bales or rolls having a consistency of 8-10%. The dry bales or rolls are subsequently defiberized using a hammermill or a pin defiberizer to form fluff. Typical products made from fluff are diapers, feminine hygiene products and incontinence products. Fluff can also be used to produce various air laid absorbent pads and paper products.
Softness is a tactile sensation perceived by the consumer holding a particular product, rubbing it across the skin or crumpling it within the hand. Softness comprises two components, bulk softness and surface softness. Bulk softness relates to how easily the paper product flexes, crumples, or otherwise yields to even delicate counter-forces. Surface softness relates to how smooth or with how much lubricity the paper product can be slid against another surface. Both of these forms of softness can be achieved by mechanical means. For example, the sheet can be calendered to flatten the crests formed when creping the sheet and improve surface softness. Through-air-drying of the sheet improves bulk softness. However, mechanical approaches by themselves are often insufficient to meet consumer softness demands.
One way to make the paper softer is to add a softening compound to the cellulosic suspension. The softening compound interferes with the natural fiber-to-fiber bonding that occurs during sheet formation in papermaking processes. This reduction of bonding leads to a softer, or less harsh, sheet of paper. WO 98/07927 describes the production of soft absorbent paper products using a softener. The softener comprises a quaternary ammonium surfactant, a non-ionic surfactant as well as strength additives. The softening agent is added to the cellulosic suspension before the paper web is formed.
A softening compound can also be applied to a dry or wet paper web e.g. by means of spraying. If the paper web is dry, the softening compound can also be printed on the paper. U.S. Pat. No. 5,389,204 describes a process for making soft tissue paper with a functional polysiloxane softener. The softener comprises a functional-polysiloxane, an emulsifier surfactant and surfactants which are noncationic. The softener is transferred to the dry paper web through a heated transfer surface. The softener is then pressed on the dry paper web. WO 97/30217 describes a composition used as a lotion to increase the softness of absorbent paper. The composition comprises an emollient which is preferably a fatty alcohol or a waxy ester. The composition also comprises a quaternary ammonium surfactant as well as one or more non-ionic or amphoteric emulsifiers.
Most softening compounds, either added to the cellulosic suspension or applied directly to the paper web, contain quaternary ammonium surfactants. Since producers and consumers are experiencing a growing environmental concern, quaternary ammonium surfactants are not always accepted. The quaternary ammonium surfactants are generally toxic to aquatic organisms and are generally considered dangerous for the environment. The quaternary ammonium surfactants can be irritating to eyes and skin, and in some cases the irritation to eyes can be severe. Thus there is clear utility in compositions that debond and soften paper products that have less deleterious effects on the environment and have improved health profiles.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.